1. Field of the Invention
Embodiments of the invention relate generally to determining resistance levels in phase change memory.
2. Description of the Related Art
Demands for memory capacity continue to increase in modern electronic devices. At the same time, size of circuitry is at an increased premium as devices which utilize memory circuitry continue to shrink. One solution as to how to increase memory capacity while minimizing the amount of space occupied by memory circuitry has involved the development of memory elements that are capable of assuming more than two states. For example, some memory elements are capable of multi-bit (e.g., more than two state) storage. For instance, rather than outputting either a high or low voltage, the memory element may output four or eight different voltage levels, each level corresponding to a different data value.
One type of multi-bit storage memory is phase change memory. Phase change memory may store multi-bit data through resistance variations caused by phase changes of the material used in the manufacture of the memory. The state of the material of which the phase change memory is composed may be switched between amorphous, partially crystalline, and crystalline states with the application of external influences, for example, heat. The different states of the material each have distinct resistance properties that may be measured and associated with a data bit.
Conventional measuring techniques fail to adequately measure the differences in the resistance values of a phase change memory. This may lead to errors in the grouping of resistance values of a plurality of phase change memory elements into a distribution curve, typically used to show the number of memory elements programmed to each of a plurality of different states. Without proper valuation of the resistance distribution of a memory array of phase change memory elements, the individual memory elements may be programmed at resistance levels that overflow and corrupt data stored at other resistance levels. Alternatively, without proper valuation of the resistance distribution of a memory array, the difference between the levels (e.g., voltage) to be sensed may be set at levels larger than required, which tends to reduce memory density and increase the cost of memory. Thus, there is a need for measuring techniques that can accurately determine the resistance ranges for phase change memory.